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6. DO ANY SPECIES BOUNDARIES....
species. Indeed, the majority of L. perenne maps have been based on a single mapping
population, the p150/112 that was created at the Institute of Grassland and Environmental
Research (IGER) and distributed to ILGI participant laboratories for genetic analysis (Jones
et al. 2002b). Two other maps published have also been developed as successors to the
p150/112 (Armstead et al. 2002; Faville et al. 2004). Given the speciation perspective all
these maps have serious weaknesses. First, being based only on L. perenne they tell nothing
about evolutionary relationships between L. multiflorum and L. perenne. Second, a very complex
descent of the p150/112 mapping population makes it more predisposed to investigation
of breeding history in parental cultivars than species history. A doubled haploid line (DH290)
derived from the Danish cultivar “Verna” was used as the female parent of the p150/112
population. The pollinator was the highly complex L. perenne hybrid derived from a cross
between a Romanian accession and a plant from a polycross of an Italian accession and
cultivar Melle (Bert et al. 1999). In summary, at least four accessions gave rise to this population
that ensures high heterozygosity and marker segregation but makes difficult evolutionary
considerations. Each accession represents its own breeding history with introgressions from
various sources and rearrangements resulting from various breeding methodologies e.g.,
anther cultures.
Therefore, the objective of the present experiments was to construct an enhanced molecular
marker-based genetic linkage map for studying genomic interactions and reproductive
barrier formation between L. multiflorum and L. perenne. The first question was whether
there are marker distortions in this interspecific cross; and, if so, what is the magnitude of
distortions. Taking into account the reasonable size of the experiment, as far as it was possible,
comparisons with intraspecific crosses were made. The second question was where
distorted markers are located and whether they are clustered in specific chromosome regions
or not.
For the first time, the mapping family was based on a F 2
population that was developed
from a single F 1
plant as it is done in the majority of crops. Genetic segregation in such F 2
s
is the result of the meiotic recombination in a single F1 genotype that ensures that only two
alleles segregate in the mapping population. In outcrossing species difficulties are encountered
in the development of such populations owing to self-incompatibility and the lack of
homozygous parental lines. Therefore all Lolium maps are constructed based on more or
less complex populations originating from several heterozygous plants. In this procedure,
apart from complicated segregations and difficulties in appropriate software usage, two independent
maps, male and female have to be constructed and further joined to produce
a sex-average map. On the other hand, even if it is time consuming, the self-pollination of
L. multiflorum and L. perenne hybrids is not impossible. Both species are not obligatory outbreeders
and various levels of self-pollination ranging from 20% to 80% have been observed
in cultivars and ecotypes (K. Polok, unpublished data). Therefore, especially in interspecific
crosses, there is no reason to use complex populations for mapping studies.
Apart from all these limitations, Lolium maps are predominantly based on RFLP as anchor
markers. Although RFLP probes are well suited for comparative mapping, their usage
encounters at least two problems. First, screening large numbers of probes across many
diverse species may be troublesome. To this point sequence tagged site markers (STS)
based on PCR reaction may be more practical. STS markers are generated by designing